1. Field of the Invention
The present invention disclosed hereinafter relates to a light-emitting element using a phosphorescent compound. Further, the present invention relates to a light-emitting device using the light-emitting element and an electronic device using the light-emitting device.
2. Description of the Related Art
In recent years, a light-emitting element using a light-emitting organic compound or inorganic compound as a light-emitting substance has been actively developed. In particular, a light-emitting element called an EL element having a simple structure in which a light-emitting layer including a light-emitting substance is provided between electrodes, has attracted attention as a next-generation flat panel display element because of its characteristics such as a thin shape, lightweight, high response speed, and direct current driving at a low voltage. In addition, a display using such a light-emitting element has a feature that it is excellent in contrast and image quality, and has a wide viewing angle. Further, since such a light-emitting element is a plane light source, the light-emitting element is considered to be applicable to a light source such as a backlight of a liquid crystal display and lighting.
In a case of using a light-emitting organic compound as a light-emitting substance, the emission mechanism of a light-emitting element is a carrier-injection type. In other words, by application of voltage with a light-emitting layer interposed between electrodes, carriers (holes and electrons) are injected from the electrodes to be recombined, and thus a light-emitting substance is excited. Light is emitted when the excited state returns to a ground state. There are two types of the excited states which are possible: a singlet excited state (S*) and a triplet excited state (T*). In addition, the statistical generation ratio thereof in a light-emitting element is considered to be S*:T*=1:3.
In general, the ground state of a light-emitting organic compound is a singlet state. Light emission from a singlet excited state (S*) is referred to as fluorescence where electron transition occurs between the same multiplicities. On the other hand, light emission from a triplet excited state (T*) is referred to as phosphorescence where electron transition occurs between different multiplicities. Here, in general, at room temperature, a compound capable of converting a singlet excited state into light emission (hereinafter referred to as a fluorescent compound) does not exhibit light emission from the triplet excited state (phosphorescence) and exhibits only light emission from the singlet excited state (fluorescence). Accordingly, the internal quantum efficiency (the ratio of generated photons to injected carriers) in a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% based on S*:T*=1:3.
On the other hand, when a compound which converts an energy difference between a triplet excited state and a ground state (a triplet excitation energy) into light emission and exhibits phosphorescence (hereinafter referred to as a phosphorescent compound) is used, internal quantum efficiency can be theoretically 75% to 100%. In other words, emission efficiency can be 3 to 4 times as much as that of the fluorescence compound. From these reasons, in order to achieve a light-emitting element with high efficiency, a light-emitting element using a phosphorescent compound has been proposed (see Non-Patent Document 1, for example). Note that in Non-Patent Document 1, an iridium complex is used, in which a ligand is 2-(2′-benzo[4,5-a]thienyl)pyridine ([btp2Ir(acac)]), as a phosphorescent compound.
The present inventors propose a light-emitting element using an organometallic complex represented by Structural Formula (50) below, which is (acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III) (abbr.: [Ir(tppr)2(acac)]) (Patent Document 1). By fabrication of a light-emitting element using the organometallic complex represented by Structural Formula (50), a light-emitting element which can exhibit red light emission with high emission efficiency can be obtained.
